According to John Chrysostom, "The Apostle acts thus so that he may not close the way to faith for the Jews by casting them into despair.
He also does this so that he may not give those who believed from the Gentiles any occasion for pride, lest they become arrogant and suffer harm in faith.
The Apostle's aim is to destroy arrogance in the believing Gentiles, and to teach the Jews that they have not fallen completely.
For as the Gentiles, having learned modesty, will remain more securely in faith, so the Jews, being led out of despair, will more readily approach grace."
Explaining the words, "through their fall salvation has come to the Gentiles," St. John Chrysostom says: "These are not Paul's words only. On the contrary, the parables in the Gospels have the same meaning.
The one who made the wedding feast for his son began to call those from the crossroads only when those invited did not wish to come (Mt 22:9).
And the one who planted the vineyard resolved to give it to other vinedressers only when the first ones had killed the heir (Mt 21:38).
The Lord Jesus came to the Jews. Yet despite the great multitude of miracles He performed, they did not receive Him and crucified Him.
After this He began to draw the Gentiles to Himself, so that by the honor shown them He might wound the senseless Jews. By mutual rivalry He would persuade them to come to Him."
As a faithful messenger of God, the Apostle himself strives to awaken in all the same spirit of rivalry, as he himself testifies in verses 13 and 14.
According to John Chrysostom, he does the same in verse 12, for "here also the Apostle says what is pleasing to the Jews.
He encourages those of them who have fallen, proving with abundance that they can hope for salvation if they change.
If, he says, so many received salvation when they stumbled, and so many received salvation when they were rejected, then consider what will be when they turn.
For he did not say, 'How much more their conversion,' or 'their change,' or 'their correction,' but, 'How much more their fullness.'
That means when all come to Christ. By this he shows that then there will be a greater, or full, measure of God's grace."
As if continuing this speech, Blessed Theodoret writes: "If, says the Apostle, when they, the Jews, did not believe, the Gentiles were received and delivered from their former ignorance, then this is clear.
If they wished to believe, nothing else would remain to be accomplished except the resurrection of the dead.
The Lord also said this: 'And this gospel of the Kingdom will be preached among all nations as a testimony to them, and then the end will come'" (Mt 24:14).